Herpes simplex viruses (HSV) are important human pathogens that cause aggressive herpetic lesions. Although current anti-HSV drugs are effective in inhibiting HSV infections, successful treatment is complicated by the increasing prevalence of drug-resistant mutants, which is becoming a clinical problem, especially in immunocompromised individuals. The development of drug-resistant mutants is a result of spontaneous mutations arising during viral replication and can be selectively amplified by anti-viral drugs. Therefore, studying the mechanism controlling the replication fidelity will lead to a better understanding of how such mutations evolve. [unreadable] [unreadable] HSV-1 DNA polymerase (pol) is the pivotal enzyme involved in DNA replication. It plays the central role in controlling the accuracy of DNA replication. Factors governing these steps are the Pol' s identity and the sequence contexts. The former includes the structure-functional properties of the Pol. The latter includes the sequence content, the positions, orientations, and replication modes of the gene being replicated. While we demonstrated that sequence contents and replication modes (plasmid- versus genome-based replication) affect replication fidelity, the effects of a gene's position and orientation in replication fidelity have not been examined. Whereas the identity of the catalytic subunit of the Pol is an important factor in replication fidelity, it appears that the UL42 subunit of the HSV-1 Pol complex also can affect the DNA replication fidelity. In this study we propose to examine the following aims. [unreadable] [unreadable] (1). Characterization of progeny derived from YD12 exonuclease-deficient pol mutants. [unreadable] (2). Roles of UL42 accessory protein of the HSV- 1 Pol in DNA replication fidelity. [unreadable] (3). Position effects of the target gene in replication fidelity. [unreadable] [unreadable] Results will be critical to the understanding of the molecular mechanisms regulating DNA replication fidelity of both HSV-1 and eukaryotes, and of the structure-functional relationship of HSV-1 Pol. This study also will be of value for the design of new antiviral drugs and treatments. [unreadable] [unreadable] [unreadable]